1. Field of Invention
The invention relates to multicolor toned electrophotographic images in which high quality colorimetric and sharpness properties are required, and are obtained using liquid toners. In particular it relates to processes of development where two or more toner images are superimposed and then transferred together to a receptor surface. Applications include the demanding area of color half-tone proofing.
2. Background of the Art
Metcalfe & Wright (U.S. Pat. No. 2,907,674) recommended the use of liquid toners for superimposed color images as opposed to the earlier dry toners. These liquid toners comprised a carrier liquid which was of high resistivity e.g. 10.sup.9 ohm.cm or more, colorant particles dipersed in the liquid, and preferably an additive intended to enhance the charge carried by the colorant particles. Matkan (U.S. Pat. No. 3,337,340) disclosed that one toner deposited first may be sufficiently conductive to interfere with a succeeding charging step; he claimed the use of insulative resins (resistivity greater than 10.sup.10 ohm.cm) of low dielectric constant (less than 3.5) covering each colorant particle York (U.S. Pat. No. 3,135,695) disclosed toner particles stably dispersed in an insulating aliphatic liquid, the toner particles comprising a charged colorant core encapsulated by a binder of an aromatic soluble resin treated with a small quantity of an aryl-alkyl material. The use of explicit dispersant additives to the toner dispersion is disclosed in U.S. Pat. No. 3,669,886.
The use of metal soaps as charge control and stabilizing additives to liquid toners is disclosed in many earlier patents (e.g. U.S. Pat. Nos. 3,900,412; 3,417,019; 3,779,924; 3,788,995). On the other hand, concern is expressed and cures offered for the inefficient action experienced when charge control or other charged additives migrate from the toner particles into the carrier liquid (U.S. Pat. Nos. 3,900,413; 3,954,640; 3,977,983; 4,081,391; 4,264,699). A British patent (GB No. 2,023,860) discloses centrifuging the toner particles out of a liquid toner and redispersing them in fresh liquid as a way of reducing conductivity in the liquid itself.
In several patents the idea is advanced that the level of free charge within the liquid toner as a function of the mass of toner particles is important to the efficiency of the developing process (U.S. Pat. Nos. 4,547,449, 4,606,989). In U.S. Pat. No. 4,525,446 the aging of the toner was measured by the charge present and related it generally to the zeta potential of the individual particles. A related patent, U.S. Pat. No. 4,564,574, of the same assignee discloses that charge director salts were chelated onto the polymer binder by specially incorporated moieties on the polymer. It further discloses measured values of zeta potential on toner particles. Values of 33 mV and 26.2 mV with particle diameters of 250 nm and 400 nm are given. The disclosed objective of that patent is improved stability of the liquid toner. Attachment of the chelated salts directly to the polymer chain necessitates the presence of the change in a random orientation off of the polymer. The charge would be generally distributed throughout the bulk and surface of the polymer. Finally in U.S. Pat. No. 4,155,862 the charge per unit mass of the toner was related to difficulties experienced in the earlier art in superposing several layers of different colored toners.
This latter problem was approached in a different way in U.S. Pat. No. 4,275,136 where adhesion of one toner layer to another was enhanced by an aluminum or zinc hydroxide additive on the surface of the toner particles.
The advantages of using binders comprising organosols (sometimes described as amphipathic particles) are disclosed in patents assigned to Philip A. Hunt Chemical Corp. (U.S. Pat. Nos. 3,753,760, 3,900,412, 3,991,226). Amongst the advantages is a substantial improvement in the dispersion stability of the liquid toner. The organosol is sterically stabilized with a graft copolymer stabilizer, the anchoring groups for which are introduced by the esterification reaction of an epoxy (glycidyl) functional group with an ethylenically unsaturated carboxylic acid. The catalyst used for the esterification is lauryldimethylamine or any tertiary amine. A similar treatment is found in U.S. Pat. No. 4,618,557 assigned to Fuji Photo Film except that they claim a longer linking chain between the main polymer and the unsaturated bond of the stabilizing moiety. Their comparative examples with the Hunt toners show that Fuji has improved the poor image quality found in the Hunt toners due to image spread, and they ascribe the improvement to the use of the longer linking chains. In both the Hunt and the Fuji patents charge director compounds when used are only physically adsorbed to the toner particles.
Diameters of toner particles in liquid toners vary from a range of 2.5 to 25.0 microns in U.S. Pat. No. 3,900,412 to values in the sub-micron range in U.S. Pat. Nos. 4,032,463, 4,081,391, and U.S. Pat. No. 4,525,446, and are even smaller in a paper by Muller et al, Research into the Electrokinetic Properties of Electrographic Liquid Developers, V. M. Muller et al, IEEE Transactions on Industry Applications, vol IA-16, pages 771-776 (1980). It is stated in U.S. Pat. No. 4,032,463 that the prior art makes it clear that sizes in the range 0.1 to 0.3 microns are not preferred because they give low image densities.
Liquid toners that provide developed images which rapidly self-fix to a smooth surface at room temperature after removal of the carrier liquid are disclosed in U.S. Pat. Nos. 4,480,022 and 4,507,377. These toner images are said to have higher adhesion to the substrate and to be less liable to crack. No disclosure is made of their use in multicolor image assemblies.